1. Field of the Invention
The present invention relates to an image processing apparatus and a method for aiding diagnosis using medical images.
2. Description of the Related Art
Change in thickness of cardiac muscle of a heart is a significant indicator in diagnosis of heart disease such as myocardial infarction.
As a method for measuring change in thickness of cardiac muscle, there is a method including acquiring time-series cross-sectional images of a heart and its vicinity; detecting an inner boundary and an outer boundary of the cardiac muscle by image processing; and the calculating thickness of the cardiac muscle in accordance with each image.
Artifacts, noise, etc. superposed on the images, however, make it difficult to extract the boundaries of the cardiac muscle automatically. Therefore, various methods for extracting the boundaries have been proposed.
U.S. Pat. No. 5,669,382 has disclosed a boundary extraction method using snake. Snake is a technique for obtaining a boundary by minimizing the value of an energy function defined on the basis of conditions such as change in luminance of an image and smoothness of a boundary surface.
When change in luminance is used, it is however difficult to detect a boundary low in luminance change.
Taking the case of a heart as an example, an endocardium, which is an inner boundary of cardiac muscle, is a boundary between cardiac muscle and blood. Accordingly, in the endocardium, luminance change is clear even on images obtained by various image diagnostic systems.
On the other hand, an epicardium, which is an outer boundary of cardiac muscle, is a boundary between cardiac muscle and a tissue surrounding the outside of cardiac muscle. Accordingly, in the epicardium, luminance change is often obscure on images. For this reason, it is difficult to extract both the endocardium and the epicardium accurately, stably and automatically.
JP-A-10-165401 has disclosed a technique in which a luminance distribution in a direction of wall thickness is calculated and a position exhibiting n % (e.g., 70 to 80%) of the maximum luminance is regarded as a boundary position.
Even in this technique, sufficient accuracy cannot be obtained because there is the possibility that the foot of the luminance distribution may be extended while it exceeds n % of the maximum luminance when luminance change in a boundary and its vicinity is obscure.